1. Field of the Invention
The present invention relates to a phosphor for use in a cathode-ray tube, particularly a color cathode-ray tube. The present invention also relates to a method of manufacturing a phosphor for use in a cathode-ray tube, particularly a color cathode-ray tube, which includes an improved phosphor surface treatment step.
2. Description of the Related Art
As is well known, green-, blue-, and red-emitting phosphors are formed into dots or stripes on a phosphor screen of a color cathode-ray tube. A slurry process is commonly used as a method of forming this phosphor screen.
The slurry process will be described briefly below. Phosphor particles are suspended in an aqueous solution mainly containing PVA and ammonium bichromate to prepare a coating slurry. A faceplate is placed in a coating machine, such as a spin coater. The coating slurry is poured onto the inner surface of the faceplate placed and spread over the entire surface. Thereafter, the faceplate is spun at a predetermined rate to spin away an excess slurry. The coated slurry is dried to form a coating film. This coating film is exposed through a shadow mask having a predetermined dot-like or stripe-like pattern. The exposed coating film is developed, and an excess phosphor layer is washed away to form a phosphor screen having the predetermined pattern on the faceplate. For a phosphor screen for a color cathode-ray tube, this process is performed in sequence for green-, blue-, and red-luminescent phosphors.
A phosphor screen formed by the use of this slurry process is generally required to have the following characteristics:
i) Dense dots or stripes are formed with an uniform film thickness. PA1 ii) Dots or stripes are formed precisely. That is, after phosphors of individual colors are coated, all dots or stripes are coated at predetermined positions with predetermined shape, width, and size. PA1 iii) Dots or stripes do not peel from a faceplate, that is binding tendency is good. PA1 iv) No cross contamination occurs. That is, a phosphor constituting dots or stripes of one luminescent component does not mix with nor overlap an adjacent phosphor of another luminescent component. PA1 v) No haze exists. That is, after an unexposed phosphor layer is washed away, no phosphor layer remains on a faceplate.
The above characteristics are influenced by the surface condition of a phosphor. For this reason, various cathode-ray tube phosphors have been developed in which various surface treatment substances are adhered or coated to phosphors to improve the surface conditions of the phosphors.
One of surface treatment substances currently most often used is silicon dioxide (SiO.sub.2 ; to be referred to as silica hereinafter). A phosphor containing silica as a surface treatment substance is commonly obtained by adding an aqueous solution containing a silicic acid ion, or ultrafine silica to a phosphor suspension, and by adding an electrolyte solution containing, e.g., a Zn or Al ion, so that these additives flocculate on the surface of the phosphor to produce a silicate compound on it.
For example, Published Examined Japanese Patent Application No. 50-15747 discloses a method of performing a surface treatment for a phosphor using zinc silicate by adding potassium water-glass and zinc sulfate to an aqueous suspension of the phosphor; and Published Examined Japanese Patent Application No. 61-46512 discloses a phosphor which is surface-treated by silica and a zinc compound.
The phosphors described in the above Published Examined Japanese Patent Applications, however, are still unsatisfactory to meet all the characteristics of items i) to v) above.
For example, adhering zinc silicate to a phosphor can improve dispersibility of phosphor particles in the photosensitive resin solution described above, thereby satisfying the characteristics of items i), ii), and iii) above. However, since phosphor particles scatter to adjacent dots, the characteristics of items iv) and v) cannot be met.
On the other hand, the phosphor which is surface-treated with water-glass and zinc sulfate described above cannot meet any characteristics of items i) to v) above satisfactorily.
As described above, the techniques of flocculating silica by using a compound of, e.g., Zn or Al to perform a surface treatment for a phosphor satisfy the characteristics of items i) to v) above to some extent and hence are most often used presently. However, as an HDTV, a high-resolution cathode-ray tube, and the like have been developed, a strong demand has arisen for a phosphor having more excellent coating characteristics.